After reading this forum post, I thought to myself that our backend hardware is similar to yours and throughput was identical.
My coluegue searched for best practices for connecting servers to iSCSI SAN's until he improved our results.
Long story short and weeks later this is everything we did to improve results:
1) He broke a 2 network cards Team we set up into 2 individual network cards.
Each card on the iSCSI network had it's on IP address.
Thus having 2 x the amount of multipath I/O connections from each server to the SAN.
2) Each IP address has permission to connect to the SAN.
3) He created multiple LUN's
4) He also broke up the single large 20TB virtual disk on the SAN into 4 smaller 5 TB disks. (This was the big one and it really improved throughput).
5) Each HyperV host has access to all 4 smaller 5 TB virtual disks on the SAN. This quadroupled the throughput via Veeam B & R.
A combination of all 5 steps above improved the results. However step 4 & 5 made all Warnings and Errors disappear from the Event Logs.
I hope I summarized it all, but after a lot of digging, this has provided a major improvement for us.